1. Field of the Invention
This invention relates to semiconductor device fabrication and, more particularly, to the production of electrical contacts to semiconductor materials.
2. Art Background
Essentially all semiconductor devices and particularly those intended for operation at high frequency, e.g., microwave frequencies, require a low resistance (specific contact resistance typically less than 10.sup.-4 ohm/cm.sup.2) electric contact to the active portions of the device. Often the method used to form the contact significantly affects the performance and reliability of the device, i.e., the percentage of devices found unacceptable after fabrication and the lifetime of the acceptable devices. For example, in the case of GaAs field effect transistors (FETs) various short-term device failure mechanisms have been associated with the contacts made to the GaAs active region. (See for example, Wemple & Niehaus, Source Drain Burn Out in GaAs MES FETS, "Gallium Arsenide and Related Compounds" (St. Louis) 1976, Institute of Physics Conference Series, 33b, 226.) Some expedients have been developed to decrease device failure induced by contact formation. In the example of GaAs FETs, N.sup.+ GaAs pads at the source and drain (as shown in FIG. 1) have been added to increase stability properties. (See 1976 Conference Series, supra, Vol. 33b, 271.)
Data on long-term stability is scarce for many devices such as power FETs. However, for many semiconductor devices, improvements such as indicated above have been reported as successful as improving short-term stability. In the case of GaAs FETs, tests have indicated a 12 volt burnout voltage for a device without N.sup.+ pads compared to a greater than 20 volt burnout voltage for a device with the N.sup.+ pads present. Despite these advances, further increase in both short- and long-term device stability and thus reliability are desirable. Additionally, when the stability of device is increased, generally the reject percentage in a fabrication line is similarly decreased. Thus, not only equipment failure, but also equipment cost is reduced by advances in the production of stable devices.